1. Field of the Invention
This invention relates generally to an apparatus for determining various physical quantities and for accomplishing non-contacting distance measurement. More particularly, the invention concerns an apparatus for determining various physical quantities such as temperature, pressure, stress, strain and the like, in a manner such that the changes in physical quantity of interest, including changes in distance, result in changes in the frequency of oscillation of a signal generated within the apparatus.
2. Discussion of the Invention
The vast majority of prior art sensing and transducing systems rely on converting the physical quantity of interest into an analog voltage or current at some stage of the signal processing. An example of this is the strain gages that are used as the transducer systems for measuring strain, pressure, force and various other physical quantities.
Such systems bring with them the inherent problems of minimizing the noise that is introduced into the signal and of determining the true signal level in the inevitable presence of at least some noise. Overcoming these problems typically results in systems that are either delicate, expensive, or both.
A limited number of other transducing systems attempt to directly measure the time delay or "time of flight" of a signal directed over some particular path in order to measure a physical property of interest. Such systems typically are used to measure distance, as in the case of radar, or electrical characteristic impedance, as can be done with a time-domain reflectometer. At the most fundamental level, these systems depend on measuring the time delay between a transmitted and a subsequently received signal.
In these systems, the necessary accuracy for the measurement of this delay can be in the sub-nanosecond range, and direct measurement of such time intervals is a substantial technical challenge. A limited number of other systems as exemplified by U.S. Pat. No. 4,885,433 (Schier) avoid the direct time interval measurement and make use of the phase difference introduced into a modulated signal by the time delay. It is of course then necessary to measure this phase difference, a process which is also sensitive to external influences and noise of various sorts. Furthermore, some systems resort to converting the time delay or phase difference into a voltage by means of an appropriate circuit, and this brings with it the problems previously mentioned.
Lastly, one small group of devices specifically converts a velocity of interest directly into a frequency by taking advantage of the doppler shift introduced by some moving object of interest. This group is typified by doppler radar systems and so-called "ring-around flow meters". These devices function by transmitting a signal at a moving target of interest (in the case of the radar) or through a moving medium of interest (in the case of the flow meter). Due to the motion of the target or the medium, the frequency of the signal is shifted, and this frequency is then detected by any of a variety of means. In many cases, and in particular in the case of doppler radar, an intermediate step of homodyne or heterodyne conversion is necessary before the frequency of interest can be detected.
In any device relying on a doppler shift, if the object of interest is receding from the transmitter the signal which is subsequently detected is lowered. Conversely, if the object is approaching the transmitter, the frequency is increased. Inevitably, such devices are adapted to measure only the velocity of the object of interest.
The thrust of the present invention is to provide an apparatus for measuring a number of different physical quantities in a manner such that a change in the physical quantity of interest results in a change in the frequency of oscillation of a signal generated within the apparatus. The invention relies on a feedback system which is made to self-oscillate at a frequency determined by the overall transmission delays in the system. The system is configured so that variation in the physical quantity of interest cause a variation in the transmission delay and consequently cause a change in the frequency of oscillation. For example, to sense the temperature a signal transmission path occupied by gas at constant pressure might be used. In this instance, as the temperature of the gas increases, the gas becomes less dense, lowering its index of refraction. This means that an electromagnetic signal transmitted through the gas will have a higher velocity in the gas, thus decreasing the overall transmission delay. This decrease in transmission delay results in a higher frequency of oscillation in the system. Another example could involve the use as a signal transmission medium of selected electro-optic materials whose properties change as a function of an external electric or magnetic field. Still another example could involve the use of an optical fiber whose index of refraction varies as a function of stress or strain applied to the fiber.